Hundreds of grades of synthetic resins for common and special applications available under various trademarks and brand names are now known in the art. Insoluble synthetic ion-exchange resins (ionites) with three-dimensional structure of macromolecules are classified, depending on the sign of the charge of exchanged ions, into cationites, anionites and amphoteric ionites. The group of ionites usually covers complex-forming sorbents absorbing substances from solutions by chemosorption, not by way of ion exchange. The process of ion exchange with participation of ionites may be regarded as a heterogeneous chemical reaction with mobile ions participating therein, while a stationary macromolecular ion of the opposite sign forms of polymeric matrix (base).
Ionites are produced mainly as spherical or irregular-shape particles. The most suitable is the spherical shape of particles, since it results in the smallest resistance to the liquid flow in columns. As regards the structural features, ionites may be classified into two types: gel-like having no transition pores, and macroporous ionites with the solid phase being pierced by pores reaching several hundreds A in their transversal dimension. The gel-like ionites are generally exemplified by regular or standard ionites, macroreticulated and isoporous. Regular solid ionites in the form of grains or granules having the gel-type structure obtained by way of polymerization or polycondensation have a swelling ability defined by the frequency and rigidity of intermolecular cross-linking bridges. In the dry and slightly swollen condition these ionites do not reveal a noticeable porosity, thus limiting their application for non-aqueous solutions, as well as for sorption of high-radius ions such as large-size organic ions.
Permeability of the polymeric matrix of an ionite is one of the most important properties thereof defining its practical use, so that in recent times great attention has been paid to the manufacture of porous ionites. Owing to a well-developed surface they are very active in reactions of polymer-analogous transformations and sorption processes; however, due to a high content of divinylbenzene (up to 50%), they have a lower exchange capacity as compared to the gel-like ionites and do not substantially swell in water.
The rate of exchange processes is substantially defined by the rate of diffusion of counter-ions to ionite grains. At a relatively large size and a comparatively small surface area of the granule surface, ion-exchange resins only slightly swell due to reticulation, whereby the diffusion process is slow and, consequently, the rate of ion-exchange is reduced.
Another disadvantage of numerous synthetic ionites resides in a usually low chemical and thermal stability, as well as a low mechanical strength. Losses of ionites due to breaking of grains along the cracks formed upon crushing of the resin, its shrinkage during heat-treatment or wetting of an ionite with water are in certain cases as high as 10-15%.
To prevent breaking of particles, a more flexible structure of ionites is required. In this connection, ion-exchange fibres acquire an ever-growing importance.
Indeed, a substantially more developed surface area of fibres, maximally enriched with active functional groups in the superficial layer, a high wettability and capillarity of fibrous materials ensures a higher rate of process thereon as compared to granulated materials. Furthermore, the effective grain size of granulated ionites is within the range of from 0.43 to 0.63 mm, whereas the transversal dimension of the majority of reactive fibrous materials is by 20-30 times smaller and ranges from 0.02 to 0.03 mm. Consequently, the path of ion diffusion in fibrous materials is also by 20-30 times shorter. This phenomenon is the main reason for a higher kinetics of reactive fibres as compared to granulated materials. It is also an important fact that the majority of modified fibres with reactive groups have a high porosity, frequently as high as 100 to 200 m.sup.2 /g. The rate of conversion of functional groups of such fibres is the highest.
However, reactive fibres, just as non-woven materials based thereon, while being packed into a column, rather rapidly become clogged thus substantially increasing the hydrodynamic resistance of the filtering layer.
To overcome this disadvantage, it is necessary that the filtering layer possess flexibility of elastic foamed plastics.
Known in the art is a process for the manufacture of polyurethane foamed plastics containing ionic groups (cf. U.S. Pat. No. 3,988,268; Cl. 260-2.5, 1976). Amphoteric foamed plastics are produced from reagents containing both cationic and anionic groups. Thus, known in the art is a process for the production of a polyurethane foamed plastic by way of reacting isocyanates such as 1-methyl-2-,4-diisocyanate with organic compounds such as ricin polyglycol ester. The mixture is heated to the temperature of 170.degree. C., maintained for three hours and then cooled to room temperature. Disadvantages of these materials may be exemplified by a low content of ionic groups, wherefore they are unsuitable for use in processes of chemisorption. They are intended mainly for a soil matrix for plant growing.
Known are also ion-exchange foamed materials (cf. U.S. Pat. Nos. 3,867,319; 1975; 3,947,387; Cl. 260-2.5 R, 1976) which are produced by foaming a polymer obtained in the presence of a volatile polar compound comprising a plastifying agent for ionic groups. The polymer contains 0.4-10 mol.% of graft acid groups, mainly sulpho groups and comprises sulphonated polystyrene. The material obtained by this process has a low exchange capacity (the number of ionic groups, in particular sulpho groups, is 0.2 to 20 mol.%) and a low mechanical strength. Furthermore, such materials feature rigidity and brittleness.
U.S. Pat. No. 3,094,494 (Cl. 260-2.1, 1963) teaches ionic cellular materials consisting of a foamed polyurethane serving as a polymeric matrix and a filler--a synthetic ion-exchange resin--in an amount of from 0.5 to 160 parts by weight per 100 parts by weight of the polymeric matrix. To produce such materials, 100 parts of a polypropyleneglycol oligomer (produced upon heating of 2 parts of a mixture of 100 g of polypropyleneglycol with the molecular mass of 2,000 and 35 parts by weight of toluenediisocyanate (isomeric mixture 80/20) are added with 67 parts by weight of a finely divided ion-exchange resin based on a sulphonic acid (sulphonated styrene and divinylbenzene in its sodium form) and agitated to a uniform composition, whereafter the mixture is added with a blend of 2.4 parts of water, 1 part of methylmorpholine, 1 part of triethylamine, 0.6 part of dimethylpolysiloxane and agitated till foaming. The foamed mixture is cast into a mould and allowed to stay there until it is completedly foamed. The materials produced by this process are flexible, resilient, gas- and liquid-permeable. The ion-exchange resin is accessible for liquids, however, under the effect of working solutions the finely-divided ion-exchange resin is washed out of the material thus impairing the exchange capacity during explotation and reducing the service life of such materials. Furthermore, the material features relatively low kinetic characteristics.
Due to such disadvantages, it is inexpedient to use ion-exchange cellular materials and resin-filled cellular foams for such applications as anti-pollution control of the environment. Owing to low values of exchange capacity, hydrophoby and a small rate of exchange processes, the use of such materials cannot ensure purification of waste waters from harmful products to required values of permissible concentrations. In addition, the possibility of varying the material characteristics for enlargement of the range of their applications is either hindered or totally eliminated.